A wide range of optical detectors have been developed to detect visible and infrared illumination, characterized by the use of a highly absorbing detector thin film or substrate [Dereniak and Boreman, “Infrared Detectors and Systems”, Wiley (1996)]. Typically, the active layer (e.g., p-n junction) of these detectors absorbs a majority of the incident photons, thereby maximizing the detection efficiency and signal-to-noise ratio. For instance, thermal detectors or bolometers absorb light within a low thermal mass structure, causing the film to heat up. The resulting temperature change is determined by measuring the resistance across a conductor spanning the structure. The conductor may be metallic, or a semiconductor/superconductor with a high temperature coefficient of resistance.
Unique structures and techniques to improve the performance of optical detectors have been reported in the literature on superconductor bolometers [A. T. Lee, P. L. Richards, S.-W. Nam, B. Cabrera, and K. D. Irwin, “A superconducting bolometer with strong electrothermal feedback,” Appl. Phys. Lett. 69, 1801 (1996)], superconductor nanowire detectors [A. Kerman, E. A. Dauler, J. K. W. Yang, K. M. Rosfjord, V. Anant, K. K. Berggren, G. N. Gol'tsman, B. Voronov, “Constriction-limited detection efficiency of superconducting nanowire single-photon detectors”, Appl. Phys. Lett. 91, pp. 101110-1 to 101110-3 (2007)] and semiconductor nanowire detectors [P. Servati, A. Colli, S. Hofmann, Y. Q. Fu, P. Beecher, Z. A. K. Durrani, A. C. Ferrari, A. J. Flewitt, J. Robertson, W. I. Milne, “Scalable Silicon Nanowire Photodetectors”, Physica E 38, pp. 64-66 (2007)].
In contrast to traditional applications of optical detection for imaging or optical signal reception, there is a specialized need in fiber optic networks to monitor high intensity optical power propagating through optical fibers in a “transparent” fashion, by development of a measurement device which leaves the optical signal substantially unattenuated. This application requires low transmission loss (<0.5 dB) moderate sensitivity (1 μW) and relatively low bandwidth (kHz). Transparency enables distributed optical monitoring across large scale fiber optic communication networks, such that multiple passes through cascaded monitors does not compromise digital and or analog optical signal quality.
Prior art optical monitors typically incorporate a semiconductor photodiode and micro-optical elements in precise alignment to divert a small port of guided light onto the photodiode. Such detectors are responsive to total optical power and are relatively independent of optical intensity. For example, US Patent Application 2009/0016716 by Isida describes a fiber array, tap coupler, photodetector, integrated with planar lightwave circuit. US 2009/0213363 by Starodubov et al. and U.S. Pat. No. 6,259,842 to Giltner utilize one or more tap beam splitters and photodiodes. Alternatively, Shapiro et al. in US 2004/0022494 describes a monitor device in which a small amount of power is tapped off onto a photodiode by selectively grinding and polishing off the optical fiber's cladding.
U.S. Pat. No. 7,042,015 to Sun et al. discloses a semi-transparent detector based on thin amorphous silicon semiconductor layer with low absorption, with a transparent conductor on top and bottom as electrical contact layers. Wang et al. describes an “Optical Power Monitor Based On Thermo-Chromic Material” in US 2009/0153837, in which photo-induced heating of an absorbing film changes color, the color change being related to light intensity.
A transmissive optical detector based on a photo-thermal-electric mechanism has been reported in U.S. Pat. No. 7,289,197, entitled “Transmissive Optical Detector”, to A. S. Kewitsch. These detectors are slightly absorptive, passing the majority of the optical signal through without degradation, and utilize transparent conductive thin films such as indium tin oxide as the sensing element.
Compact, low loss fiber optic components incorporating transmissive detector elements within industry standard fiber optic interfaces, using differential configurations for compensation for ambient temperature changes, will enable the automation of optical network management and testing for improved network visibility and safety. In particular, safety and automation are increasingly important considerations in designing systems based on optical fibers. Advances in the design and manufacturing of fiber-coupled laser sources are leading to a proliferation of mainstream high power fiber applications and raising new laser safety issues. Numerous laser applications in communication networks, manufacturing and medicine have created a demand for more power, primarily to reduce the cost to transmit data over increasing distances or to speed-up or enable new processes and procedures.
It is common for the optical power carried by optical fibers to exceed eye safe limits. In particular, Raman amplified fiber optic communication systems are widespread and transmit high optical powers (>1 W) within single mode fiber. In addition, 1.0 μm fiber lasers producing 1 to 10,000 W are used in a wide range of manufacturing, medical and defense applications to perform processes such as cutting, marking, printing and welding. In these systems the fiber optic beam delivery systems typically propagate optical power away from the laser source and deliver light to a distant target through detachable connectors and cable segments. As a result, the users of such a system are often unaware that the fiber is transmitting significant power, leading them to disconnect or handle the active fiber in an unsafe manner.
For instance, if the fiber is bent excessively, harmful levels of optical power can escape from the side of the fiber due to bend-induced outcoupling, analogous to a leaky pipe. Since infrared wavelengths are invisible to the human eye, the natural protective reflexes of the eye (i.e., squinting) do not occur. To remedy this safety hazard in a manner than is compatible with existing fiber optic systems, a small form-factor indicator of unsafe optical power levels is needed. Current photodiode-based tap couplers are not well suited for this application because they are bulky, high loss and costly. Therefore, there is a further need for new devices and systems to enhance the intrinsic safety of fiber optic systems.